On January 2, 2020, the Ministry of Industry and Information Technology
issued specifications for the comprehensive utilization of used batteries,
proposing to establish a comprehensive traceability management platform and
conduct full-process information collection on a series of processes including
battery production, use, scrapping, recycling, and utilization. At the same
time, the new national standard for waste battery recycling requires battery
manufacturers to "sell one and collect one", and the battery industry is
ushering in a huge change.
Battery industry experts believe that the reshuffle of the power battery
industry will accelerate, and a complete improvement in the industry will have
to wait until the explosive consumption of new energy vehicles occurs in the
later period. In addition, some leading battery companies have been making
frequent moves recently and are exploring new application areas such as electric
ships.
Optimal control of hydrogen concentration in the fuel cell anode is one of
the key performance parameters that affects the efficiency and durability of
fuel cell electric vehicles. Installing a hydrogen concentration sensor is a
common practice for hydrogen concentration management in the anode of fuel cell
systems. However, the current hydrogen concentration sensor chip has not yet
overcome the impact of moisture and the technology is not mature enough. The use
of hydrogen concentration sensors is extremely limited. Therefore, it is
necessary to develop a new hydrogen concentration estimator to maintain the
hydrogen concentration in the fuel cell hydrogen system pipeline (including the
stack anode chamber) at a certain level. This article shares Hyundai Motor
Company's NEXO fuel cell anode hydrogen concentration estimator and
estimator-based purge controller.
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maximum discharge rate: 3C -40℃ 3C discharge capacity retention rate ≥70%
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Vehicle fuel cell systems usually include fuel cell stacks, air supply
systems, hydrogen supply systems, and thermal management systems. To improve
fuel cell efficiency and durability, material breakthroughs and technology
updates are essential. Among them, the most effective but dangerous technology
is the optimization of anode hydrogen concentration. Due to the limitations of
hydrogen concentration sensors for fuel cell vehicles in terms of accuracy,
life, cost, reliability and other factors, in order to maintain the hydrogen
concentration at a certain level during fuel cell operation, the widely used
method is the equivalent Q value (EQV) method. That is, the output current that
changes with time in the fuel cell is multiplied and integrated by the weighting
factor, as shown in the following formula.
Hyundai NEXO fuel cell system architecture
When the equivalent Q value is higher than the target level, the anode
performs a purge operation to increase the hydrogen concentration. However, this
method is an indirect method to maintain the hydrogen concentration within a
certain range, and is greatly affected by environmental conditions and driving
conditions. To deal with these uncertainties, Hyundai Motor Company has done a
lot of work to revise the weighting factors in the equivalent Q-value method.
Therefore, to obtain the true anode hydrogen concentration that is closest to
what is measured by the hydrogen sensor, Hyundai developed the Hydrogen
Concentration Estimator (HCE) based on thermodynamics, fluid dynamics,
mathematical modeling, and sensors within the fuel cell system. In addition,
based on the hydrogen concentration estimator, Hyundai Motor Company designed
and developed anode voltage regulation controller and purge controller to
achieve optimal control operation of hydrogen concentration, and conducted a
large number of experiments on fuel cell vehicles.
Hydrogen concentration control design concept
The usage rate of hydrogen from on-board hydrogen storage bottles is mainly
divided into three types, one is the available usage rate, and the other two are
unavailable usage rates. First, the available usage rate is the hydrogen usage
(more than 90%) used by the fuel cell to generate electricity to drive the
vehicle forward and operate the system-related actuators, defined as the amount
of hydrogen required to generate electricity/total hydrogen amount.
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The first type of hydrogen unavailability is related to the diffusion of
hydrogen across the proton membrane to the cathode (hydrogen infiltration),
where the cathode hydrogen flows with air to the air outlet manifold.
Theoretically, the amount of hydrogen permeation is proportional to the
difference in hydrogen partial pressure between the cathode and anode. The
greater the pressure difference, the greater the amount of hydrogen permeation.
The nitrogen that diffuses from the cathode to the anode across the proton
membrane will be recirculated through the hydrogen supply system pipeline (such
as a hydrogen circulation pump), and will evolve over time, causing the hydrogen
concentration in the hydrogen system pipeline to decrease. Therefore, the
accumulated nitrogen in the hydrogen pipeline should be removed. Discharge to
the air outlet manifold to increase the anode hydrogen concentration, an
operation called purging. But during purge, hydrogen will also be discharged
together with nitrogen and water vapor, which is the second source of
unavailable hydrogen usage. In order to improve efficiency, the above two
unavailable hydrogen usage rates need to be minimized, and it is not advisable
to maintain the hydrogen concentration (hydrogen partial pressure) in the
hydrogen supply system pipeline at an ultra-high level. However, the large power
generation command triggered by the driver will result in the inability to
supply enough hydrogen (the partial pressure of hydrogen is low), thereby
permanently damaging the fuel cell electrodes. Therefore, it is particularly
necessary to maintain a higher hydrogen concentration range to prevent damage,
but at the same time, hydrogen penetration will also increase, resulting in
reduced efficiency. There are compromises and trade-offs between efficiency and
durability. To achieve both high efficiency and high durability of fuel cells,
the hydrogen concentration should be optimally controlled, which requires
embedding a real-time hydrogen concentration estimator in the hydrogen supply
system.
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